1. Field of the Invention
This invention relates to a compact Keplerian finder optical system of variable magnification.
2. Related Background Art
In recent years, in the field of lens shutter cameras for example, cameras containing photo-taking lenses of variable focal length therein have become the mainstream. Along therewith, finders of continuously variable magnification have been required. Particularly Keplerian finders have been enjoying increasing demand as finders for high-class cameras because the finder field is divided clearly by a field frame and ghost and flare are insignificant. Therefore, in recent years, various Keplerian finder optical systems of variable magnification have been proposed, as for example, in U.S. Pat. No. 4,842,395 and Japanese Laid-Open Patent Application No. 1-116616.
However, the Keplerian finder optical system adopts a system in which a real image formed by an objective lens is enlarged and observed through an eyepiece, and this has led to the disadvantage that dust adhering to a field frame or a lens disposed near it, and bubbles or other defects in the lens, are also enlarged and seen at the same time. To overcome this disadvantage, it will suffice if nothing is disposed near the focal plane of the objective lens, but if this is done, the feature of the Keplerian finder that the field of view is divided clearly will be eliminated. When conventional real image type finders of simple construction are viewed from such a point of view, any one of them has required a so-called field lens near the focal plane of the objective lens and no consideration for coping with the above-noted disadvantage has been given.
Further, in recent years, the plastic molding technique has made rapid progress and now almost all of the optical parts of finders from aspherical lenses to prisms of complicated shapes can be provided by molded plastic articles. However, it is difficult to avoid minute dust or bubbles mixing with molded plastic articles during the molding thereof, and where a field lens or the like disposed near the focal plane is provided by a molded plastic article, dust in the field lens is conspicuously observed and this has remarkably reduced the rate of non-defectiveness of finders.
Also, mentioning a problem in aberration correction, particularly the finder proposed in the aforementioned U.S. Pat. No. 4,842,395, which comprises a few constituents, has a difficulty in the correction of the fluctuation of distortion, and among finders having a field of view of 50.degree. or more at the wide angle end, there has been none in which distortion is corrected well.